Heads up. After six-and-a-half years circling the Earth, China's first space station is predicted to re-enter the atmosphere in early April – give or take a week. The presently unmanned space laboratory Tiangong-1, which lost contact with mission control in 2016, is being tracked by space agencies around the world and is expected to break up over the Earth's surface somewhere between 42.7° North latitude and 42.7° South latitude.

It's difficult to appreciate the difficulty of the job that space scientists and engineers do until something goes so fundamentally wrong that all they can do is sit back and watch how things play out. Tiangong-1 is an example of this. As it circles the Earth at an altitude of about 260 km (162 mi), the 10.5-m (34-ft) long, 8,500-kg (18,740-lb) space station, its orbit has been steadily decaying since its last altitude adjustment on December 16, 2015.

Normally, this wouldn't be a problem. There have been large spacecraft that have reentered the Earth's atmosphere and burned up in the past without incident. The largest was the 120-tonne Russian Mir space station, which burned up over the South Pacific on March 23, 2001. However, the policy of all space agencies and militaries is to make sure that any spacecraft that is big enough for fragments to reach the ground should make a controlled entry so they break up over an uninhabited ocean region.

Tiangong-1 is the exception. Launched on September 30, 2011 from the Jiuquan Satellite Launch Center, China, atop a Long March 2F/G rocket, the two-module space habitat was designed to accommodate two astronauts. In 2012, it was visited by the three-astronaut Shenzhou 9 mission, which included China's first female astronaut, and in 2013 by Shenzhou 10.

It was a demonstrator unit for the next, larger Tiangong station, scheduled to launch in 2023, but things went wrong in March of 2016 when the Chinese National Space Administration announced that it had lost telemetry contact with Tiangong-1. The agency is notoriously secretive, so it isn't known for sure whether the lab is still under ground control, but since no course corrections have been made, it is highly probable that the spacecraft is inactive and amateur astronomers claim that it has been dormant since June 2016.

What this means is that there is a high degree of uncertainty as to when or where Tiangong-1 will enter the atmosphere. Solar activity can cause the upper layers of the atmosphere to expand, causing unpredictable increased drag on low-orbiting satellites, which means that timing the end of the station's life has a very large margin of error of 20 percent.

Worse, the area overflown by Tiangong-1 amounts to two-thirds of the Earth's surface and it is uncertain how much, if any, of the station will survive reentry. The biggest fear is that some of the attitude thruster fuel tanks containing toxic hydrazine may survive, posing a hazard to the general public. However, experts at Aerospace, a research center for the United States Air Force, say that the odds of anyone being injured by fragments is one million times less than winning the Powerball lottery jackpot.

"In terms of where it will land, a few hours of uncertainty spans a lot of territory," says Jay Melosh, a professor of earth, atmospheric and planetary sciences at Purdue University. "That could be the difference between landing in Chile and the middle of the Pacific."

For the moment, we'll just have to wait and see.

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